The Environment Agency’s environmental risk assessment for decamethylcyclopentasiloxane (D5) is based on the methods outlined in the European Union (EU) Technical Guidance Documentfor the risk assessment of new and existing chemicals. The persistence, bioaccumulative, and toxic (PBT) status is assessed, and a ‘quantitative’ risk assessment made by comparison of exposure with effects. Persistence, bioaccumulative, and toxic status D5 meets the screening criteria for a very persistent (vP) and very bioaccumulative (vB) substance. It is unlikely to meet the screening criteria for persistent organic pollutants in long-range transport. Laboratory studies indicate that D5 is not readily biodegradable in aquatic systems. However, it is difficult to interpret some of the results because of the rapid loss of D5 through volatilisation. A standard test modified to prevent such loss gives a hydrolysis pH-dependent half-life of 71 days at pH 7 and nine days at pH 8 (both at 25°C). The equivalent half-life at pH 7 and12°C is estimated to be around 315 days, and those at higher pHs (e.g. around 8, as can occur in the marine environment) and 9°C are estimated as 64 days. The final products of the hydrolysis of D5 are not thought to have PBT properties. The lack of biodegradation in laboratory tests and the relatively slow rate of hydrolysis at pHs around 7 mean D5 meets the persistent and vP criteria for water. Although, the volatility of D5 affects its residence time in water–sediment systems, and is probably the predominant removal mechanism for D5 from water, adsorption onto sediments also occurs. D5 lost to air because of its high volatility undergoes subsequent degradation in the air. The bioconcentration factor (BCF) for BCF of D5 in fish is 7060 l/kg (determined experimentally). In addition, D5 is accumulated by fish from diet, and a growth-corrected and lipid-normalised biomagnification factor (BMF) of 3.9 is derived from the available experimental data. Thus, D5 meets the vB criterion. D5 shows essentially no acute toxicity to aquatic organisms when tested at concentrations up to its water solubility limit, as do the limited long-term toxicity data available. In addition, D5 is not classified as a carcinogenic, mutagenic, or reprotoxic compound. Based on these data D5 does not meet the toxic criterion. However, the available long-term fish toxicity data may not cover all of the relevant toxicological endpoints, so it is not fully established whether or not D5 has the potential to cause effects in fish over long-term exposure. For example, a recent accumulation study with fish shows only slow depuration of accumulated D5 from the liver, and the long-term impact of the accumulation in liver of fish is not known. In addition, effects on liver weight occur in rats at relatively low doses of D5. However, it is not clear if these effects alone are sufficient to warrant D5 as toxic.